Titanium and its alloys are highly reactive with oxygen and will rapidly form an oxidized layer on exposed surfaces when exposed to air or moisture of any kind. This means that titanium and its alloys will reoxidize very quickly even after the removal of an oxidized layer. This makes the formation of coatings or layers of other types of materials directly onto titanium or titanium alloys difficult and layers formed on an oxidized titanium layer may not be uniform because of lack of adhesion.
The formation of a platinum (Pt) passivating film on titanium nitride (TiN) is of value to the memory applications. Because of the rapid oxidation of TiN after removal of the native oxide a vacuum deposition of Pt onto TiN has been attempted using physical vapor deposition techniques. The use of a vacuum atmosphere solves the problem of reoxidation of the TiN in air but PVD is a nonselective deposition, requiring the subsequent removal of platinum from regions other than the TiN and therefore increasing the number of processing steps. Wet processes to deposit platinum on TiN are inherently difficult to use because TiN bared of its oxide will rapidly oxidize when exposed to water or air in such a non-vacuum environment. The oxide layer may be removed before the application of the wet processes such as electrodeposition or electroless deposition. The oxidized layer is typically removed using an acidic formulation that includes hydrofluoric acid (HF). This is detrimental to most substrates because HF is a strong etchant and will attack materials other than the oxide layer. In particular, in memory applications, HF will attack materials such as silicon, silicon dioxide, and tungsten. Therefore HF chemistries make the use of subsequent wet processes difficult. Also, similar to PVD, electrochemical deposition of Pt is nonselective to TiN.